Motor vehicle electrical systems may be configured as so-called two-voltage or multi-voltage vehicle electrical systems including at least two subsystems. Such electrical systems are used, for example, when consumers having different power requirements exist in a particular motor vehicle. In this case, at least two of the subsystems have different voltage levels, for example, 14 V (a so-called low-voltage subsystem) and 48 V (a so-called high-voltage subsystem). The subsystems may be connected to each other, for example via a DC-DC converter. At least one of the subsystems has a generator system that feeds the subsystem. A second or additional subsystem connected via the mentioned DC-DC converter may then in turn be supplied from the subsystem having the generator system.
Electric machines may be used, in particular, in hybrid vehicles in order to be motor operated as well as generator operated. The internal combustion engine may be assisted by a motor operation of the electric machine at low rotational speeds at which the former does not yet deliver its full torque. Upon deceleration of the motor vehicle, kinetic energy may then be converted into electrical energy by the generator operation of the electric machine.
During generator operation, the electric machine generates, if necessary, a polyphase current which may be rectified for a motor vehicle electrical system. To enable both motor operation as well as generator operation of the electric machine, the electric machine may be equipped with an inverter circuit which may be composed, for example, of electrical switches, for example, in the form of MOSFETs, an associated control circuit and an intermediate capacitance. To ensure high performances in both motor as well as generator operation of the electric machine, the electric machine may be operated with, or it may supply, the comparatively high, first voltage of the high voltage subsystem.
However, the use of both an inverter circuit and a DC-DC converter in this configuration is cumbersome and is associated with high costs. Moreover, the separate circuits of the inverter circuit and the DC-DC converter put a strain on the already severely limited installation space in a motor vehicle.
It is therefore desirable to provide a simple, cost-efficient and space-saving option for enabling both a generator as well as a motor operation of an electric machine in conjunction with different subsystems of the motor vehicle electrical system.